The stator bar windings of generators of a certain size are typically water-cooled. That is, water flows from an inlet coolant water header into flow passages within the hollow copper strand stator bars and then flows outwardly into an outlet coolant header for flow into a reservoir. The coolant water is supplied to the windings via a closed loop system including a heat exchanger and a deionizer. Leaks in the stator windings of water-cooled generators, for example, at the brazed joints of the windings, can eventually cause insulation damage that can affect the reliability and longevity of the unit. Early detection of such leaks enables strategic testing and repair to be scheduled during minor outages, avoiding costly surprise replacements and extended outages. If early insulation damage is not discovered, the problem can quickly compound itself, as stator components are subject to thermal shock, cycling, corrosion and mechanical vibrations. This harsh environment causes and exacerbates leaks at a variety of locations, though most commonly at series loops and other brazed connections. These leaks will ultimately lead to a catastrophic in-service failure of the generator and forced removal from service if allowed to persist and grow unchecked.
It will be appreciated that in water-cooled generators, a cooling core hydrogen environment is normally maintained at a higher pressure than the coolant water flowing through the stator windings. This pressure difference, combined with stator component permeability of the Teflon flexible hoses causes a slight, barely detectable, flow of hydrogen into the coolant water under normal operating conditions even in a leak-free generator. However, when leaks actually develop, the quantity of hydrogen flowing into the coolant water increases slowly at first and at a dramatically increased rate as the leak path enlarges. By continuously or periodically monitoring the leakage flow of hydrogen into the coolant water, upward trending or step increases in the volume of hydrogen leakage can be used as a reliable indicator of water leaks and the potential for electrical insulation damage.
An additional concern involves the oxygenation level of the coolant water. With proper aeration, a tenacious and protective cupric oxide film advantageously forms on the inside surfaces of the copper windings. However, when the coolant water oxygen level drops too low, approximately 1 part per million (ppm), a less stable cuprous oxide layer is formed along these surfaces. This layer tends to break away from the winding surface, sloughing off base copper and introducing particles into the system. Oxygenation of the coolant water for generators is currently provided by air exchange through a vent line from the coolant water storage tank or reservoir to the atmosphere. Unfortunately, air in this line is relatively stagnant and the typical long length of the line, upwards of hundreds of feet in some installations, makes oxygen exchange difficult. Furthermore, significant hydrogen leaks may cause a constant outward flow of gas through this line, thus totally isolating the water from fresh air.
According to prior U.S. Pat. No. 5,492,004, there is provided a stator leak monitoring system which gives an on-line indication of a generator stator bar insulation condition, i.e., a winding leak, by measuring the volume of hydrogen escaping from the generator core into the stator bar water cooling system. The system also simultaneously oxygenates the coolant water by flowing air into the coolant water. More particularly, the stator leak monitoring system disclosed in the '004 patent, measures the volume of hydrogen that leaks from the generator core into the stator coolant water and uses this data as an indication of potential stator bar insulation damage. To accomplish the foregoing, an opening to the atmosphere is provided in the closed coolant water system adjacent the top of the generator where the coolant water exits the stator windings. At this location, coolant water flowing downwardly into the water reservoir creates a low-pressure area that induces an inward flow of air, i.e., air is aspirated into the coolant water. The exit flow through the vent for the reservoir is unidirectional but is not stable enough to be measured. Thus, air flow into the system is measured and the percentage of H2 in the gas exiting through the vent is determined whereby the total volume of H2 leaking out the vent can be ascertained. More specifically, the '004 patent discloses that the volume of hydrogen leaking or escaping from the generator core can be determined as follows:
      H          2      ⁢              (        VOL        )              =                    %        ⁢                  H          2                            1        -                  %          ⁢                      H            2                                ⁢          (      Inflow      )        ⁢          (      k      )      
where
% H2 is the fraction of H2 measured in the gas exiting the reservoir vent to atmosphere; Inflow is the rate of fresh air flowing into the system; and k is a conversion factor.
The inflow of air provides fresh air to mix with the coolant water as the water returns to the reservoir. This ensures that the coolant water has sufficient oxygen levels to avoid undesirable oxide formation on the winding surfaces. By locating a hydrogen gas analyzer for sampling gas flowing through the reservoir vent and locating a gas (air) flow meter at the inlet opening for the air into the system, the quantity of hydrogen in the vented gas stream can be measured as a percentage of total flow. Thus, the escaping hydrogen volume may be determined and the data interpreted as an indication of cooling system leaks.
Successful stator water leaks monitoring requires an accurate determination of the existence of a water leak from inception, and the ability to trend/track the progress of that leak(s). That is not done well currently, where a leak must be several times background levels of hydrogen permeation through Teflon hoses to establish that a true leak exists. Further in this regard, existing operating guidelines determine if there is a leak and whether it is minor, significant or major according to the following criteria:
Less than 3 cu. ft./day, considered background and not necessarily a water leak;
Greater than 3 and less than 10 cu. ft./day, one or more leaks present and maintenance required at next outage.
Greater than 10 and less than 30 cu. ft./day, significant leak or multiple leaks present with repairs recommended within 1 year or less.
Greater than 30 and less than 200 cu. ft./day, major leak present most likely from plumbing piping failure. Monitor continuously and repair ASAP.
Greater than 200 cu. ft./day, high possibility of major in-service failure, remove from service immediately.
More sensitive monitoring is desirable to more accurately identify leaks and the progress of such leaks over time.